Photolithography is a fabrication technique that is employed for use in a number of industries, including the semiconductor processing industry. Specifically, photolithography exposes energy, such as ultraviolet (UV) light, x-ray wavelength, other wavelengths of radiation, etc. to selected regions of a surface. In one common technique, the surface includes a semiconductor wafer such as silicon that has been coated with a resist material. The resist material properties are locally changed when exposed to the energy source, which allows selected regions of the resist material to remain, while unwanted regions of the resist material are removed.
In one method of photolithography, a pattern of features is formed on a patterning tool, such as a reticle or mask, (a patterning tool is referred to hereinafter in the specification by example as a “reticle”) and energy transmitted through the pattern on the reticle is focused onto a working surface to print functionally important features on the working surface using a lens that adjusts the scale of the pattern on the reticle to fit the working surface. In the semiconductor industry, there is an ever present pressure to reduce the size of features in the pattern to increase the density of printed features packed into the same semiconductor surface area. In one example industry, manufacturers of memory chips such as dynamic random access memory (DRAM) or flash memory strive to put more storage cells onto a single chip.
As feature size decreases, photolithography of smaller and smaller features becomes more and more difficult. Imaging fidelity can be degraded by some non-ideal properties of a lens, such as aberrations. During high volume manufacturing, a lens constantly receives energy and lens heating may cause optical aberrations. Methods and devices to reduce lens heating induced optical aberrations are needed to keep pace with ever increasing photolithography demands such as smaller feature sizes.